This invention relates to synchronous data transmission systems and, more particularly, to a method and to an apparatus for quickly determining the initial values of the coefficients of a transversal equalizer in such a system.
In synchronous data transmission systems, the message to be transmitted is in the form of a bit sequence and is first converted into a sequence of symbols, where each symbol can take one of a discrete number of values which number is generally selected as equal to a power of two. The symbols are then sent over a transmission channel at a given rate, called signaling rate, in the form of pulses that may or may not be modulated depending on whether or not the transmission system uses a carrier modulation technique. Generally, the transmission channels, and more particularly, the telephone lines, introduce amplitude and phase distortions that modify the shape of the signals. These distortions are generally due to the imperfect transfer characteristics of the transmission channels involved and are aggravated by the noise introduced therein by external sources. The control of such distortions and noise presents varying degrees of difficulty, for example, the amplitude and phase distortions can create an interaction between successive signals. This interaction, known as intersymbol interference, can preclude reliable detection of the data signals at the receiver. In the higher speed data transmission systems, the receivers are generally provided with a device designed to minimize the effects of intersymbol interference before the data are detected. Such a device is called an equalizer.
The type of equalizer which is the most widely used at the present time is the automatic transversal equalizer described, for example, in "Principles of Data Communication," by R. W. Lucky, J. Salz and E. D. Weldon, Jr., Chapter 6, McGraw-Hill, New York, 1969. Such an equalizer consists of a transversal filter whose coefficients are automatically adjusted to meet a given performance criterion. In general, during an initial period referred to as a training period, a set of isolated test pulses or a pseudo-random training sequence is transmitted to allow the coefficients of the equalizer to be adjusted to initial values that are as close as possible to optimum values. Subsequent to the training period, the initial values are not further adjusted during transmission of the message itself unless an adaptive type of equalizer is used.
If the distortion characteristics of the transmission channel vary between successive messages, which may be the case where the channel consists of public telephone lines, a training period must be provided before transmitting each message. However, the efficiency of a data transmission system is usually measured by the ratio of the time interval required for completing transmission of the message to the occupancy time of the line, the latter time interval corresponding essentially to the duration of the training period plus that of the transmission of the message. If the efficiency level is to remain at a suitable level in a high speed data transmission system, in which transmission of a message is typically completed within a few tens of milliseconds, provision must be made for a device capable of minimizing the duration of the training period, that is to say, capable of determining as quickly as possible the initial values of the equalizer's coefficients.
The determination of the initial values of the equalizer's coefficients, referred to hereafter as initial equalization, can be carried out either in the time domain or in the frequency domain.
The initial equalization technique as carried out in the time domain is discussed in an article by K. H. Mueller and D. A. Spaulding entitled "Cyclic Equalization--A New Rapidly Converging Equalization Technique For Synchronous Data Communication," in The Bell System Technical Journal, pp. 369-406, February 1975, and in French Patent application No. 75 40417 filed by the assignee of the present applicant on Dec. 30, 1975, and corresponding to U.S. patent application, Ser. No. 743,022, filed Nov. 18, 1976, by A. T. Milewski now U.S. Pat. No. 4,089,061. While the method described in said patent application is quite effective, it has been found that the speed at which the initial equalization is performed could be increased by applying said method to the frequency domain.
The frequency-domain initial equalization technique has been discussed in many publications, particularly in an article by Tien-Lin Chang entitled "A New Equalization Technique," in Proceedings of the National Electronics Conference 1973, pp. 335-338, Chicago, Illinois., October 1973. The method described by Tien-Lin Chang is fast and gives satisfactory results in the absence of noise, but is ineffective when the transmission channel is noisy, and even more so when frequency shift is introduced by the channel. As is known, a considerable amount of frequency shift is introduced by most transmission channels, and especially by those which are used in long distance communication links.